On the stability of cerium inhibiting layers on aluminium 2024-T3: an in situ optical and electrochemical study

Abstract

Salts of trivalent cerium (Ce(III)) are known to successfully hinder localised corrosion phenomena typical of aerospace aluminium alloys by forming a thin passivation layer on top of the corrosion-active intermetallic phases. This is generally observed to happen when the alloy is under immersion conditions in a solution containing Ce3+ ions. However, some very preliminary works suggest that protection provided by this layer is lost once the passivated metal is re-exposed to a corrosive solution in absence of inhibitor (for example, aqueous solution of sodium chloride). This (in)stability process has not been directly addressed in literature and is, therefore, not well understood. A deeper understanding of such process and the strategies that could lead to a higher passive layer stability are needed to bring Ce-based inhibitor technologies closer to application and replacement of the harmful Cr(VI)-based primers. In this work we studied the inhibition mechanism and stability of cerium inhibiting layers created on AA 2024-T3 upon exposure to electrolytic solutions containing Ce(NO3)3 and NaCl. The stability of Ce(III) layers after re-exposure to corrosive electrolytes (i.e. without inhibitors) is investigated. Furthermore, the interaction between Ce(III) layers and different organic compounds, namely 2,5-Dimercapto-1,3,4-thiadiazole (DMTD), phytic acid and sodium alginate, was also explored as a potential strategy for stabilisation of Ce-based inhibitive layers.
Due to the complex interpretation of electrochemical signals typically used in corrosion studies (e.g. electrochemical noise and impedance spectroscopy), it was decided to combine such electrochemical methods with highly spatio-temporally resolved in-situ microscopy in order to assess both corrosion of AA 2024-T3 and its inhibition. For this purpose, a home made optical-electrochemical experimental setup was used, together with a protocol for optical data analysis. This hyphenated in-situ protocol is further supported with SEM-EDS, Raman and SKPFM analysis after exposure. In the work here presented, the unstable nature of Ce-inhibitive deposits was found to be associated to their dissolution after re-exposure to inhibitor-free electrolytes. Studies devoted to the stabilisation of Ce(III)-inhibitive layers with organic molecules indicated different behaviours and outcomes. Alginate oligomers efficiently passivate the Ce-protected intermetallic phase, but, at the same time, destabilise the Al-rich matrix, which became subject to general corrosion. Phytic acid was found to provide synergy with Ce(III) layers, which however lacked in stability. Treatment of Ce-passivated surfaces with DMTD, on the other hand, shows a significantly delayed onset of optical-electrochemical activity upon re-immersion in NaCl solution, which suggested the accomplishment of a more stable system. The results obtained confirm the power of in-situ (i.e. under immersion) optical investigation not only to support the understanding of complex electrochemical signals, but also to deliver quantitative information relevant in corrosion and inhibition studies. The approach proposed in this research is believed to be of future help for an improved design of Cr(VI)-free anticorrosion coating systems.